Processes for the treatment of a sour hydrocarbon fraction where the fraction is treated by contacting it with an oxidation catalyst and an alkaline agent in the presence of an oxidizing agent at reaction conditions have become well known and widely practiced in the petroleum refining industry. These processes are typically designed to effect the oxidation of offensive mercaptans contained in a sour hydrocarbon fraction to innocuous disulfides, a process commonly referred to as sweetening. The oxidizing agent is most often air. Gasoline, including natural, straight run and cracked gasolines, is the most frequently treated sour hydrocarbon fraction. Other sour hydrocarbon fractions which can be treated include the normally gaseous petroleum fractions as well as naphtha, kerosene, jet fuel, fuel oil, and the like.
A commonly used continuous process for treating sour hydrocarbon fractions entails contacting the fraction with a metal phthalocyanine catalyst dispersed in an aqueous caustic solution to yield a doctor sweet product. Doctor sweet means a mercaptan content in the product low enough to test "sweet" (as opposed to "sour") by the well-known doctor test. The sour fraction and the catalyst containing aqueous caustic solution provide a liquid-liquid system wherein mercaptans are converted to disulfides at the interface of the immiscible solutions in the presence of an oxidizing agent--usually air. Alternatively, the sour hydrocarbon fraction may be effectively treated by contacting it with a metal chelate catalyst dispersed on a high surface area adsorptive support--usually a metal phthalocyanine on an activated charcoal at oxidation conditions in the presence of a soluble alkaline agent. One such process is described in U.S. Pat. No. 2,988,500. The oxidizing agent is most often air admixed with the fraction to be treated, and the alkaline agent is most often an aqueous caustic solution charged continuously to the process or intermittently as required to maintain the catalyst in the caustic-wetted state.
The prior art shows that alkaline agents are necessary in order to catalytically oxidize mercaptans to disulfides. Thus, U.S. Pat. Nos. 3,108,081 and 4,156,641 disclose the use of alkali hydroxides, especially sodium hydroxide, for oxidizing mercaptans. Further, U.S. Pat. No. 4,913,802 discloses the use of ammonium hydroxide as the basic agent. U.S. Pat. No. 5,232,887 discloses the use of solid base materials which are used both as the support for the metal catalyst and as the alkaline agent. The activity of the metal chelate systems can be improved by the use of quaternary ammonium compound as disclosed in U.S. Pat. Nos. 4,290,913 and 4,337,147.
We have developed a catalytic system of solid materials and a process using the catalytic system which is significantly different from all the sweetening processes previously disclosed in the art. The prior art describes numerous types of oxidation catalysts used in combination with an alkaline agent. Furthermore, the prior art systems disclose the catalyst in intimate contact with the alkaline agent. In contrast, our invention involves the use of a solid base which is not required to be in intimate contact with the oxidation catalyst. In fact, our invention provides that the oxidation catalyst and alkaline agent be physically separated into two reaction beds. Moreover, the demonstrated high conversion of mercaptans to disulfides of our invention was contrary to expectations set by the generally accepted working hypothesis of how the alkaline agent functions and by mercaptan oxidation of kerosine studies using the oxidation catalyst alone and the solid base alone.